1. Field of the Invention
The present invention relates to a gas sensing member which is used for a gas sensor disposed in an exhaust system of an internal combustion engine of a vehicle and detects a concentration of a specific component included in a measured gas output from the engine, and also relates to a method of manufacturing the gas sensing member.
2. Description of Related Art
A gas sensor with a gas sensing member is disposed in an exhaust system of an internal combustion engine of a vehicle. The gas sensing member is formed in a rectangular parallelepiped. The member has a plate-shaped solid electrolyte body having upper and lower surfaces opposite to each other along a lamination direction of the member, a gas measurement electrode disposed on the upper surface of the electrolyte body and exposed to a measured gas, a reference gas electrode disposed on the lower surface of the electrolyte body and exposed to a reference gas, and a heater disposed under the lower surface of the electrolyte body to heat the body to an activity temperature of the body. The electrolyte body has a high electric conductivity for oxygen ions.
In this sensing member, when the operation of the engine is started, the heater heats the electrolyte body to an activate temperature of the body to activate the body. When an exhaust gas output from the engine flows through the exhaust system, the gas measurement electrode is exposed to the exhaust gas, and oxygen ions obtained by decomposing the exhaust gas or the reference gas pass through the electrolyte body from one of the electrodes to the other electrode. Therefore, an electric potential difference is generated between the electrodes. Then, a concentration of a specific component (e.g., O2 or NOx) included in the exhaust gas is detected from the electric potential difference.
The sensing member further has a gas inlet through which the exhaust gas is entered into a gas chamber. The gas measurement electrode is exposed to the exhaust gas in the gas chamber. The exhaust gas has poison which adversely influences the gas measurement electrode. To protect this electrode from the poisons, the whole outer surface of the sensing member is covered with a porous protective layer so as to cover the gas inlet with the protective layer. The exhaust gas is transmittable through the protective layer. When the exhaust gas passing through the gas inlet is entered into the gas chamber, the poisons of the exhaust gas are caught or trapped by the protective layer disposed on the gas inlet. Therefore, the measures gas electrode is exposed to the exhaust gas substantially not having poisons, so that the electrode can be protected from the poisons of the exhaust gas.
To form the protective layer on the sensing member, a distal portion of the sensing member to be exposed to the exhaust gas is dipped into a slurry solution wherein ceramic particles are mixed with solvent, and the slurry solution adhering to the sensing member is dried. Therefore, a layer of ceramic particles is attached to the whole surface of the distal portion of the sensing member. A proximal portion of the member is inserted into a holder of a gas sensor, so that the proximal portion is not exposed to the exhaust gas. The proximal and distal portions of the member are aligned with each other along a longitudinal direction perpendicular to the lamination direction.
Further, drops of water are produced in the engine due to combustion of fuel and are inevitably included in the exhaust gas as moisture. During the detection in the gas sensing member, the drops of water fly with the exhaust gas and are attached on the outer surface of the porous protective layer. The porous layer has a high water holding property, so that the drops of water attached to the protective layer easily adhere to and penetrate into the protective layer. Therefore, the temperature at a portion of the protective layer receiving the water is locally lowered. Particularly, when the drops of water are attached to a portion of the protective layer placed on the heater and reach the outer surface of the heater which is maintained at a high temperature, the temperature of the surface of the heater is rapidly lowered, and a large difference in temperature is generated between a water receiving area and an area surrounding the water receiving area on the surface of the heater. Therefore, thermal stress is generated in the heater, and there is a probability that the heater will be cracked or broken due to this thermal stress. For example, heating elements disposed in the heater are occasionally broken or disconnected from a power supply line.
More specifically, the heater is formed in a thin plate shape and is placed at the bottom of the sensing member. The heater has a heater substrate and a plurality of heating elements buried in the heater substrate. The heating elements are aligned with one another in the distal portion of the member along a width direction perpendicular to the lamination and longitudinal directions. Each heating element extends along the longitudinal direction. The heater substrate has side corner portions on both sides in the width direction. The side corner portions are placed at bottom side corners of the sensing member. When drops of water are attached on a particular area of the protective layer just placed on one side corner portion of the heater substrate, thermal stress is caused on the side corner portion of the heater substrate. Because the side corner portion of the heater substrate is placed at a corner of the sensing member, the thermal stress is easily concentrated on a narrow area. Therefore, there is a high probability that the heater is cracked or broken at its side corner portion.
For example, Published Japanese Patent First Publication No. 2003-322632 discloses a gas sensing member. In this Publication, to prevent breakage of the sensing member caused by drops of water attached on the outer surface of the sensing member, the whole surface of the distal portion of the member is covered with a porous protective layer. However, because the porous layer has a high water holding property, drops of water attached on the outer surface of the protective layer easily penetrate into the protective layer and reach the outer surface of the sensing member to lower the temperature of the sensing member. Particularly, when drops of water are attached on a particular surface area of the protective layer placed on a side corner portion of a heater, a thermal stress is concentrated in the side corner portion, and the heater is easily broken or cracked. Therefore, the protective layer disposed on the side corner area of the heater does not protect the heater, but rather heighten a probability of the breakage of the heater.
To prevent drops of water attached on the surface of the protective layer from reaching the surface of the sensing member, there is an idea that a protective layer disposed on the sensing member is thickened. In this case, the drops of water attached on the surface of the protective layer are dispersed in the protective layer along directions parallel to the surface of the protective layer. Therefore, the drops of water do not reach the sensing member, and breakages and cracks of the sensing member can be prevented. However, the thickened protective layer increases a thermal capacity of the sensing member. In this case, when the operation of the engine is started, it takes a long time to heat the sensing member, and the electrolyte body of the sensing member cannot rapidly reach its activity temperature. Therefore, when the operation of the engine is started, it becomes difficult to accurately detect a concentration of a specific component included in the exhaust gas, and it is very likely that fuel will not completely combust in the engine.